How the Blow-Up Ratio (BUR) Affects Blown Film Quality

The Unseen Force in Film Quality: Understanding the Blow-Up Ratio

For many newcomers to the film blowing industry, the relationship between the size of the die, the width of the film, and the ultimate strength of the final product can seem like a dark art. A common and frustrating problem is producing a film with drastically different properties along its length and width, leading to issues like poor sealing and unreliable bags. The key to mastering this lies in understanding a critical process parameter: the Blow-Up Ratio (BUR).

This article will demystify the blow-up ratio, explaining what it is, how it's calculated, and, most importantly, its profound effect on the quality and performance of blown film.

What is the Blow-Up Ratio (BUR)?

In simple terms, the blow-up ratio is a measure of how much the plastic tube is expanded, or "blown up," after it exits the circular die. It is calculated by dividing the final diameter of the bubble by the diameter of the die.

BUR = Final Bubble Diameter / Die Diameter

For practical purposes on the factory floor, the final bubble diameter can be determined from the lay-flat width of the collapsed film:

Final Bubble Diameter ≈ Lay-flat Width x (2/π) or Lay-flat Width x 0.637

Therefore, a common practical formula is:

BUR = (Lay-flat Width x 0.637) / Die Diameter

For example, if a 100 mm die is used to produce a film with a lay-flat width of 471 mm, the BUR would be approximately 3:1. This ratio is a fundamental control lever that dictates the orientation of the polymer molecules, which in turn defines the film's mechanical properties.

How the Blow-Up Ratio (BUR) Affects Blown Film Quality

The Balancing Act: How BUR Dictates Film Properties

The process of blowing the film stretches the polymer chains in two primary directions: the machine direction (MD), which is the direction the film is pulled, and the transverse direction (TD), which is across the width of the film. The blow-up ratio is the primary factor that determines the balance of orientation between these two directions.

Blow-Up Ratio	Effect on Molecular Orientation & Film Properties
Low BUR	- Predominant orientation in the machine direction (MD). - Higher MD Tensile Strength: The film is stronger and stiffer along its length. - Lower TD Tensile Strength: The film is weaker and more prone to splitting across its width. - Higher MD Tear Resistance: More difficult to initiate a tear along the length. - Lower TD Tear Resistance: Easier to tear across the width.
High BUR	- Greater orientation in the transverse direction (TD). - Lower MD Tensile Strength: The film is less strong along its length. - Higher TD Tensile Strength: The film is stronger and stiffer across its width. - Lower MD Tear Resistance: Easier to tear along the length. - Higher TD Tear Resistance: More difficult to tear across the width.

The "Securely Sealed Bag" Problem: The issue of bags that are not sealed securely often stems from a significant imbalance in the film's tensile properties. If the film has a very high MD strength and a very low TD strength (a result of a low BUR), the heat-sealing process can be compromised. The weaker TD direction may not have the integrity to form a strong, reliable seal, or it may be prone to splitting near the seal when under stress. Achieving a more balanced film by optimizing the BUR is crucial for applications requiring strong seals.

BUR's Influence on Other Key Film Qualities

Beyond tensile strength, the blow-up ratio impacts several other critical film attributes:

Impact Strength: Generally, a higher BUR can lead to improved impact strength, as the biaxial orientation of the polymer chains creates a more robust structure that can better absorb and distribute impact forces.
Optical Properties: Increasing the BUR often enhances the film's clarity and reduces haze. The stretching of the film in two directions can lead to a smoother surface and a more uniform internal structure.
Film Thickness and Uniformity: While the final film thickness is also controlled by the extrusion speed and take-up rate, the BUR plays a role. A higher BUR will naturally result in a thinner film for a given die gap and output. Maintaining a stable bubble, which is directly related to the BUR, is also essential for achieving uniform thickness across the film's width.

Typical Blow-Up Ratios for Common Polymers

The ideal BUR is not a one-size-fits-all number; it is highly dependent on the type of polymer being processed due to its unique molecular structure and melt characteristics. Here are some general guidelines:

Low-Density Polyethylene (LDPE): Typically runs at a BUR of 2:1 to 4:1. LDPE is quite forgiving and produces a stable bubble over a relatively wide range of ratios.
Linear Low-Density Polyethylene (LLDPE): Often processed in a similar range to LDPE, from 2:1 to 4:1. Achieving the optimal BUR is key to maximizing LLDPE's renowned toughness and puncture resistance.
High-Density Polyethylene (HDPE): Requires a higher BUR, often in the range of 3:1 to 6:1, especially for high molecular weight (HMW) grades. The higher ratio is necessary to achieve a good balance of stiffness and tear resistance.

Conclusion: From Novice to Knowledgeable

The blow-up ratio is far more than just a simple calculation; it is a cornerstone of the film blowing process. By understanding and controlling the BUR, operators can move from being at the mercy of their machinery to being in command of their film quality. Knowing your die size is only the first step. The ability to manipulate the BUR to achieve the desired film width and, more critically, the optimal balance of mechanical properties, is what separates inconsistent output from high-quality, reliable film production. The next time a bag seal fails, a closer look at the blow-up ratio may just provide the answer.